The present invention relates to an improvement of nickel positive electrode for alkaline storage batteries and a method for producing the same.
Recently, there is an increasing demand for secondary batteries having a high energy density with the development of compact and light-weight portable appliances with high added-values. It has also been desired to develop a novel secondary battery with a high energy density as a power source for electric vehicles. In order to serve the growing need for such secondary batteries, high capacity batteries using a conventional sintered nickel electrode have been developed in the technical field of nickel-cadmium battery. In parallel to this, batteries with a high energy density using a foamed metal type nickel positive electrode having a high capacity by 30 to 60%, and even nickel-metal hydride storage batteries using a hydrogen storage alloy for the negative electrode have also been under development. The latter is not less than two times higher in capacity than the nickel-cadmium battery using the sintered type nickel positive electrode.
In the above-listed high capacity alkaline storage batteries, a nickel hydroxide powder is filled in high density into a sintered nickel porous substrate, a three-dimensional foamed nickel porous substrate with a porosity of as high as not less than 90%, or a nickel fiber porous substrate in order to improve the energy density of the positive electrode. This increases the energy density of the positive electrode to 450 to 500 mAh/cm3 with the currently used sintered type nickel positive electrode, and to even 550 to 650 mAh/cm3 with the foamed metal type nickel positive electrode, compared to the value of 400 to 450 mAh/cm3 with the conventional sintered type nickel positive electrode.
However, positive electrodes prepared by filling a nickel hydroxide active material in high density into a sintered nickel porous substrate, a foamed nickel porous substrate, or a nickel fiber porous substrate have a drawback that their energy density decreases at high temperature, although it is high around room temperature. This may be because under the charging condition of high temperature atmosphere, oxygen evolution reaction is liable to occur simultaneously with the charging-associated reaction where nickel hydroxide is oxidized to nickel oxyhydroxide. This means that in high temperature environment, the oxygen evolution potential decreases at the positive electrode, resulting in insufficient oxidation of nickel hydroxide to nickel oxyhydroxide, thereby decreasing the utilization of nickel hydroxide.
In order to solve this drawback, there is a suggested method to incorporate at least one selected from the compounds of yttrium, indium, antimony, barium, calcium and beryllium into the positive electrode (see, Japanese Laid-Open Patent Publication Hei 5-28992). Any of these compounds incorporated into the positive electrode is adsorbed onto the surface of the nickel hydroxide active material and acts to improve the utilization of the nickel hydroxide active material during charging in a high temperature atmosphere. However, there still exists a demand for further increased utilization in high temperature atmosphere.
On the other hand, there is another proposal to improve the utilization of nickel hydroxide by a method of forming cobalt hydroxide on the surface of nickel hydroxide active material and then heating it in the presence of oxygen and an alkaline aqueous solution to form a highly conductive compound of cobalt with a high valence over 2 (higher than 2 valent) on the surface of the nickel hydroxide active material (Japanese Laid-Open Patent Publication Hei 1-200555). Although this method is effective for improving the utilization of the nickel hydroxide active material around room temperature, the effect on the utilization in a high temperature atmosphere is not large. Now, those two references are incorporated herein by reference.
From the above, the object of the present invention is to provide a nickel positive electrode for alkaline storage batteries exerting an exceptional utilization of nickel hydroxide active material in an environment from room temperature to high temperature.
The present invention provides a nickel positive electrode for alkaline storage batteries comprising particles of a nickel hydroxide material, a coating layer comprising a compound of cobalt with a mean valence over 2 and coating the particles, and a compound of at least one metallic element selected from the group consisting of Ca, Sr, Ba, Cu, Ag, Cd, Y, Yb, Ce, Sm, Gd and Er.
Here, as the compound of at least one metallic element, one selected from the group consisting of Ca(OH)2, CaO, CaF2, CaS, CaSO4, CaSi2O5, calcium oxalate CaC2O4, CaWO4, SrCO3, Sr(OH)2, BaO, Cu2O, Ag2O, CdO, Y2O3, Y(OH)3, Y2(CO3)3, Yb2O3, Yb(OH)3, CeO2, Sm2O3, Gd2O3 and Er2O3 is preferred.
While the novel features of the invention are set forth particularly in the appended claims, the invention, both as to organization and content, will be better understood and appreciated, along with other objects and features thereof, from the following detailed description taken in conjunction with the drawings.